Photo-ionization detectors (PIDs), flame-ionization detectors (FIDs) and related volatile gas detection devices are well-established, commercially available product designs capable of measuring the total airborne concentration of volatile organic compounds (VOCs) present in the inlet air of the device. Such devices are offered by a number of manufacturers. For example, RAE Systems offers a range of PIDs, and Photovac and Thermo Fisher each offer both a PID and an FID. Some of these devices rely on the ionization properties of VOCs to create a measureable signal that can be correlated to the concentration of these compounds.
The PID related volatile gas detection devices sample air continuously, using a pump to induce a precisely regulated flow through the device. Hence, the measurement of total VOC concentration can be in real-time and continuously updated. Modern PIDs are typically equipped with a computer interface, including a physical data port and a set of programming instructions. This allows the user to create custom software that operates the PID and acquires VOC measurements and other data from the PID.
As an adjunct to the PID, a pre-filter can be employed to prevent interfering VOCs from entering the inlet airflow of the PID, permitting only a specific VOC, or set of VOCs, of interest to pass into the PID. The pre-filter is placed in the inlet air stream of the PID so that the filtering action occurs prior to the measurement of the air sample. This, in turn will permit the PID's measurement of a VOC to reflect only the concentration of the specific compound of interest. Such pre-filters are commercially available, for example from Draeger or RAE Sytems, and are used to monitor the concentration of benzene or other substances of interest. These pre-filters physically trap other, interfering VOCs that are not to be measured, preventing those compounds from being detected by the PID.
Nevertheless, there are challenges and constraints associated with the proper usage of pre-filters. Such pre-filters typically have a finite service life and will eventually become invalid after prolonged exposure to the atmosphere, due to saturation by VOCs and to effects of temperature and humidity. They are thus perishable items that must be replaced when no longer suitable for service. The duration of the valid service life of a given pre-filter will depend on various factors, such as the specific chemical constituents and their concentrations in the ambient air to which the pre-filter is exposed, the cumulative volume of air that flows through the pre-filter, the duration of exposure of the pre-filter, and the ambient temperature and humidity.
U.S. Pat. No. 5,654,498 discloses a device for the selective detection of a component in a gas mixture, or a pre-filter that is useful in the various aspects of the present invention.